The invention is in the gallium nitride semiconductor field.
Heterostructure field effect transistors (HFETs) are electronic devices having three terminals including a gate, a drain and a source. An electric potential applied to the gate terminal controls the flow of current from the drain terminal to the source terminal via an electrically conductive channel. The electrically conductive channel is defined by at least one heterointerface between two different semiconductor materials. If at least one of the two semiconductor materials includes GaN or an alloy of GaN with Indium or Aluminum, the device is referred to as a GaN-based HFET.
Often HFETs fabricated with different material systems, such as AlGaAs/GaAs materials, further include a barrier layer disposed between the channel layer and a buffer layer that electrically isolates the channel layer from the buffer layer thereby eliminating a number of non-ideal and generally undesired HFET behaviors. For example, the barrier layer, by preventing the flow of electrons from the channel layer to the buffer layer, reduces the number of electrons that may become trapped in the buffer layer. In addition, the barrier layer suppresses the flow of leakage current, reduces buffer layer related output conductance and improves pinch-off characteristics. It can also improve the high speed characteristics of the HFET, particularly ft and fmax.
Unfortunately, attempts to produce GaN-based HFETs having a conventional barrier layer have been unsuccessful. Specifically, the barrier layer in an HFET is conventionally formed using an alloy containing the same semiconductor material present in the channel and further containing aluminum. Thus, GaN-based HFETs having a conventional barrier layer would include a layer of AlGaN disposed between the channel and the buffer layers. However, using AlGaN to form a barrier layer in a GaN-based HFET leads to interface roughness, and frequently contamination with impurities such as oxygen. Further, AlGaN when used as a barrier layer, contains polarization charges caused by electrical properties that are inherent to AlGaN (spontaneous polarization) and further caused by electrical properties resulting from strain associated with forming the AlGaN layer (piezoelectric polarization). These polarization charges result in the formation of electrical fields that cause the semiconductor device to exhibit undesirable, non-HFET behavior characteristics. As a result, a GaN-based HFET having a conventional, aluminum doped barrier layer is not feasible.
HFETs formed with GaN materials typically include a barrier layer of AlGaN that is disposed on the channel layer to induce a high concentration of electrons in the channel and thereby enhance the electrically conductive properties of the channel. Unfortunately, the AlGaN barrier disposed on top of the channel makes ohmic contact with the channel difficult. In addition, the polarized nature of the AlGaN layer disposed on top of the channel results in the formation of surface charges that adversely affect the operation of the GaN-based HFET. Further, HFETs formed with an AlGaN layer on top of the channel layer suffer from trapping effects wherein electrons migrate from the channel to the AlGaN layer and become trapped.
Thus, there is a need for a GaN-based HFET structure that addresses some or all of the aforementioned difficulties.
A GaN-based HFET of the invention includes a barrier layer that is disposed between a buffer layer and a channel layer. Polarization charges associated with the barrier layer create a potential barrier that prevents electrons from flowing out of the channel and into the buffer.
In another embodiment, an inverted GaN-based HFET includes a barrier layer disposed between the buffer layer and the channel layer but does not require a barrier layer disposed on top of the channel. Polarization charges associated with the barrier layer create a potential barrier that prevents electrons from flowing out of the channel layer and into the buffer layer. The barrier layer and the buffer layer may be doped in a manner that induces a desired level of electron concentration in the channel. Alternatively, the channel layer may be doped in manner that induces a desired level of electron concentration in the channel layer.